The invention relates to a moisture sensor comprising a thin layer formed from the oxide of highly resistive porous low density tantalum applied on a moisture-insensitive substrate and at least two electrodes provided at the tantalum oxide layer in spaced-apart relationship, and a method of manufacturing such a moisture sensor.
A moisture sensor of this type is disclosed in commonly assigned U.S. patent application Ser. No. 272,065, filed June 9, 1981, now U.S. Pat. No. 4,433,319. In most of the embodiments of the moisture sensor described in said patent application one of the two electrodes is a base electrode disposed between the substrate and the tantalum oxide layer. Said base electrode in all instances is formed jointly with the tantalum oxide layer by oxidizing a layer of low density tantalum only over part of its thickness so that below the tantalum oxide layer there remains a portion of metallic low density tantalum forming the base electrode. This mode of forming the base electrode is possible with the low density tantalum mentioned in the elder application having a resistivity in the order of up to 40,000 .mu..OMEGA.cm. At the density and porosity of the low density tantalum corresponding to this resistance value controlled oxidation down to the desired depth is possible.
However, meanwhile it has become possible by suitable selection of the parameters in sputtering the low density tantalum to reach resistivity values in the order of 1 .OMEGA.cm and more. A low density tantalum of this resistivity is so porous that the depth of oxidation, especially when the latter is effected by anodizing, can no longer be controlled down to a desired depth; rather does the low density tantalum oxidize uniformly and simultaneously throughout the film thickness. The production methods described in the elder application then do not leave a base electrode.
The invention has as its object an improvement of the moisture sensor of the elder application so that it can be provided in a simple way with a base electrode even if the tantalum oxide layer is formed from low density tantalum of extremely high resistivity.
According to the invention this is accomplished in that the base electrode disposed between the substrate and the tantalum oxide layer consists of valve metal of a density higher than the density of the low density tantalum from which the tantalum oxide layer is formed.
As is well known, the term "valve metal" is used for anodically oxidizable metals which include especially tantalum, aluminum, niobium and hafnium. The use of such a metal for the base electrode which metal, however, must have a density higher than that of the low density tantalum layer from which the moisture-sensitive tantalum oxide layer is formed, offers the advantage that with the measures of the method described in the elder application the tantalum oxide layer can be formed by oxidation (especially anodic oxidation) of low density tantalum of extremely high resistivity while the base electrode remains intact. While the oxidation of the extremely high-resistive low density tantalum takes place simultaneously throughout the film thickness, the metal of the base electrode is continuously oxidized proceeding from the surface, the depth of oxidation being determined by the oxidation parameters. In this way the oxidation process can be reliably controlled without the base electrode disappearing. The oxide layer forming from the metal of the base electrode offers the additional advantage that it may act as insulation against the required counter-electrode. Therefore, the counter-electrode may be applied (especially in the form of a covering electrode) directly onto the oxidized structure.
Special advantages are attained when the metal used for the base electrode is also tantalum, yet of a higher density than that of the low density tantalum layer used for the formation of the moisture-sensitive tantalum oxide. Then it is possible to sputter the two layers in substantially a single operation in the same apparatus by merely selecting the sputtering parameters so as to obtain the desired different densities.
Since the moisture-sensitive layer of the moisture sensor of the invention can be made from extraordinarily porous tantalum oxide, it exhibits very high sensitivity.
By way of a further improvement the rate of response of the moisture sensor can be substantially increased if it is provided with a covering electrode partially covering the tantalum oxide layer and having windows through which the water vapor containing medium can penetrate into the moisture-sensitive tantalum oxide layer. Only the regions of the tantalum oxide layer disposed between the electrodes are active in measuring the moisture so that the water vapor must first penetrate through the inactive regions disposed below the windows in order to reach the active regions. According to the invention, the rate of response of the moisture sensor is substantially increased by removal of the inactive regions of the tantalum oxide layer below the windows. Preferably the inactive regions are removed by plasma etching with the use of the covering electrode as etching mask.
Advantageous modifications of the moisture sensor and methods of manufacturing the same are characterized in the subclaims.
Further advantages and features of the moisture sensor according to the invention and of the method of manufacturing the same will be apparent from the following description of an example with reference to the drawings.